Title

Author

Date of Award

Document Type

Degree Name

Doctor of Philosophy (PhD)

Department

Psychology & Neuroscience

First Advisor

Richard Olson

Second Advisor

Akira Miyake

Third Advisor

Al Kim

Fourth Advisor

Matt Keller

Fifth Advisor

Phillip Gilley

Abstract

Learning to read is one of the most crucial academic milestones young children face. Even with consistent instruction, however, some children excel and become good readers, while other children struggle. While most previous research in early reading development has been phenotypic, the three studies in this thesis used data from the International Longitudinal Twin Study (Byrne et al., 2009) to explore the genetic and environmental etiologies underlying individual differences in learning to read. The first study fit longitudinal data on three measures of reading and one measure of spelling from twins in Colorado to biometric growth curves. The results revealed that individual differences at the intercept (post-1st grade) and on rates of growth through the end of fourth grade were largely due to genetic influences, rather than shared environmental influences. The second study expanded upon the first by including twins from Australia and Scandinavia, and by measuring growth from the end of kindergarten through the end of second grade. The differences in intercept (post-kindergarten) etiologies between countries were consistent with the hypothesis that starting formal literacy education reduces the amount of environmental variance in a sample, thus increasing the proportion of variance due to genetic factors. Individual differences in growth on reading, in contrast, had large genetic estimates in all samples. While the first two studies focused on variance as children start to learn to read, the third study explored the etiology of the covariance between pre-reading skills and early reading development. This study found that genetic influences on print knowledge, rapid naming, phonological awareness, vocabulary, and phonological memory were predictive of early reading acquisition, even for pre-reading skills that had large shared environmental influences. Taken together, the results of the studies suggest that genetic influences are the primary reason why children in these samples varied in their early reading development. While it is important to note that these results may not generalize to all populations or to a specific child, they are an important contribution to the reading literature. Implications of these findings, including for future interventions and public policy, are discussed.